Your best bet, to start with, would be to try and boot in safe mode (F8 on boot, usually). Secondly, when you say he can't select any users, does this mean he cannot move his mouse or select them with his keyboard etc.? You may want to check both items are firmly plugged in. Otherwise, I guess we'll just have to wait and see.

Wow, the dedicated team work and double-posting that is the TechNation team

Just one little point to add to what's already been said: If the mouse is the problem, you should be able to select users using either the arrow keys, or <tab>. This should cycle between them.

There's also a slight possibility that you might be able to get the "traditional" login screen up, try pressing <Ctrl+Alt+Del> on the welcome screen. This used to work, though I have a feeling that it doesn't any more. To login from here you need to username of the user to get in (and note that this may be different from the displayed name of the user account on Welcome).

Finally, if it's USB peripherals then you should be able to unplug/plug them on the welcome screen and have them reinstalled without having to reboot. Again, this is a bit touchy but it could offer up something if you swapped the mouse into a different USB port.

There's also a slight possibility that you might be able to get the "traditional" login screen up, try pressing <Ctrl+Alt+Del> on the welcome screen. This used to work, though I have a feeling that it doesn't any more.